chemistry : class11 Wetting and Non-Wetting : Surface tension : States of Matter: Gases and Liquids MCQ Questions and Answers
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Wetting and Non-Wetting
MCQ questions and answers.
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MCQ Questions
1. A liquid completely wets a clean solid surface when the contact angle at the solid–liquid–air interface is:
A.
Exactly 180°
B.
Greater than 90° but less than 180°
C.
Exactly 90°
D.
Less than 90°, approaching 0° for complete spreading
Correct Answer:
Less than 90°, approaching 0° for complete spreading
Explanation:
Wetting occurs when adhesive attraction between the liquid and solid is sufficiently strong compared with cohesive attraction within the liquid. This produces a small contact angle; complete wetting corresponds to a contact angle approaching 0°.
2. Water forms a concave meniscus in a clean glass capillary, whereas mercury forms a convex meniscus. Which comparison correctly explains this difference?
A.
Adhesion of water to glass exceeds water's cohesion, while mercury's cohesion exceeds its adhesion to glass
B.
Cohesion of water exceeds its adhesion to glass, while glass has no adhesion with mercury
C.
Both liquids have greater adhesion to glass than cohesion, but mercury has a lower density
D.
The meniscus shape is determined only by liquid density and is independent of adhesion and cohesion
Correct Answer:
Adhesion of water to glass exceeds water's cohesion, while mercury's cohesion exceeds its adhesion to glass
Explanation:
Water wets glass because water–glass adhesive forces exceed water's cohesive forces, giving a concave meniscus. Mercury does not wet glass because mercury's cohesive forces exceed its adhesion to glass, producing a convex meniscus.
3. For a liquid to rise spontaneously in a narrow capillary tube, which condition is necessary at the liquid–solid–air boundary?
A.
The contact angle must be greater than 90°
B.
The contact angle must be less than 90°
C.
The surface tension must be zero
D.
The liquid must have greater density than the solid
Correct Answer:
The contact angle must be less than 90°
Explanation:
A contact angle less than 90° indicates wetting. The upward vertical component of surface tension then acts along the capillary wall and can raise the liquid column.
4. A liquid has a contact angle of 120° with a solid surface. Which observation is most consistent with this contact angle?
A.
The liquid spreads into a thin film and rises in a capillary
B.
The liquid forms nearly spherical drops and is depressed in a capillary
C.
The liquid forms a perfectly flat layer with no surface tension
D.
The liquid spreads completely because its contact angle is greater than 90°
Correct Answer:
The liquid forms nearly spherical drops and is depressed in a capillary
Explanation:
A contact angle greater than 90° indicates non-wetting. The liquid tends to minimize contact with the solid, forms a convex meniscus or rounded drops, and experiences capillary depression.
5. If the adhesive force between a liquid and a solid is increased while the liquid's cohesive force remains unchanged, what is the expected effect on wetting?
A.
The contact angle increases and wetting becomes weaker
B.
The contact angle decreases and wetting becomes stronger
C.
The contact angle becomes exactly 180°
D.
Wetting becomes impossible because surface tension must increase
Correct Answer:
The contact angle decreases and wetting becomes stronger
Explanation:
Stronger adhesion between the liquid and solid favors spreading over the solid surface. Consequently, the contact angle decreases, indicating improved wetting.
6. A small drop of liquid X spreads over a solid surface, while an equal drop of liquid Y contracts into an almost spherical shape on the same surface. Which inference is most appropriate?
A.
Liquid X has stronger solid–liquid adhesion relative to its cohesion than liquid Y
B.
Liquid Y has stronger solid–liquid adhesion relative to its cohesion than liquid X
C.
Liquid X must have greater density than liquid Y
D.
Liquid Y must have zero cohesive force
Correct Answer:
Liquid X has stronger solid–liquid adhesion relative to its cohesion than liquid Y
Explanation:
Spreading indicates wetting, which occurs when adhesion to the solid is strong relative to cohesion within the liquid. A nearly spherical drop indicates comparatively weak wetting.
7. For a liquid in a capillary tube, the vertical component of surface tension at the contact line is downward rather than upward. Which situation does this indicate?
A.
Complete wetting and capillary rise
B.
Partial wetting with a contact angle less than 45°
C.
Non-wetting and capillary depression
D.
Zero contact angle and zero capillary effect
Correct Answer:
Non-wetting and capillary depression
Explanation:
When the contact angle exceeds 90°, the vertical component of surface tension directed along the wall is downward. This is characteristic of non-wetting liquids and causes capillary depression.
8. A liquid wets a solid surface only partially, producing a contact angle of 60°. Which statement is correct?
A.
The liquid is non-wetting because its contact angle is not zero
B.
The liquid wets the surface and will show capillary rise, although not complete spreading
C.
The liquid must show capillary depression because its contact angle is less than 90°
D.
The adhesive force is necessarily zero because the contact angle is finite
Correct Answer:
The liquid wets the surface and will show capillary rise, although not complete spreading
Explanation:
A contact angle between 0° and 90° represents partial wetting. The liquid still wets the surface and rises in a capillary, but it does not spread completely as it would when the angle approaches 0°.
9. Why does a liquid that does not wet the material of a capillary tube generally form a convex meniscus?
A.
The liquid–solid adhesive attraction is weaker than the liquid's cohesive attraction
B.
The liquid–solid adhesive attraction is stronger than the liquid's cohesive attraction
C.
The liquid has no intermolecular cohesive forces
D.
The solid surface has no effect on the liquid's surface tension
Correct Answer:
The liquid–solid adhesive attraction is weaker than the liquid's cohesive attraction
Explanation:
In a non-wetting system, molecules of the liquid attract one another more strongly than they are attracted to the solid. The liquid therefore pulls away from the wall, producing a convex meniscus.
10. A detergent is added to water placed on a greasy surface. The water spreads more readily after addition. In terms of wetting, the most suitable explanation is that the detergent:
A.
Increases water's cohesive force so that drops become more spherical
B.
Makes the surface completely non-wetting by increasing the contact angle
C.
Improves contact between water and the surface by reducing the effective resistance to spreading
D.
Eliminates adhesion between water and the greasy surface
Correct Answer:
Improves contact between water and the surface by reducing the effective resistance to spreading
Explanation:
Detergents promote wetting by helping water spread over surfaces that it would otherwise wet poorly. They reduce the tendency of water to remain in isolated drops and improve liquid–surface contact.
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